The present invention relates to automatic transmission control method and system having a fail-safe function. Specifically, the present invention relates to automatic transmission control method and system capable of performing fail-safe operation upon occurrence of failure in a hydraulic control device for friction elements of an automatic transmission such as a hydraulic control valve.
Conventional automatic transmission control systems are adapted to detect occurrence of electric failure in solenoids, sensors and the like in a hydraulic control circuit and perform fail-safe control depending on the electric failure detected. However, failure of automatic transmissions includes not only the electric failure but also mechanical failure of a hydraulic control valve for controlling a hydraulic pressure that is supplied to friction elements. For instance, the mechanical failure would be caused due to sticking of a valve body of the hydraulic control valve or lodging of a foreign object into the hydraulic control valve.
If the friction element to be engaged is caused to be in a disengagement state due to the mechanical failure of the hydraulic control valve, a gear stage different from instructed one will be established or a neutral stage will be established contrary to the instruction, namely, so-called neutral-fail will be caused, to thereby make it impossible to restart a vehicle after the stop. Occurrence of the operating failure of the hydraulic control valve can be readily detected. However, in contrast to the case of the electric failure, it is difficult to determine the friction element to which the operating failure relates.
Japanese Patent Application First Publication No. 2003-269602 discloses an automatic transmission control system in which when failure such as the neutral-fail is detected, a vehicle is temporarily stopped, and then an engagement command will be generated for engaging friction elements so as to establish a third speed in a low-speed range for the sake of vehicle emergency traveling in the low-speed range. In the automatic transmission control system of this conventional art, if the neutral condition is continued even after the engagement command is generated, then another engagement command will be output for engaging friction elements so as to establish a fifth speed in a high-speed range for the sake of vehicle emergency traveling in the high-speed range. In the conventional art, it is intended to enable the vehicle to restart by temporarily selecting any one of the gear stages even though it is not possible to determine the friction element to which the failure relates.